Oxide-based ceramics have many applications as structural or functional materials. Ultrafine, monodispersed oxide powders are important building-block materials for fabrication of advanced ceramics. The characteristics of powders, in terms of size shape/morphology, monodispersity, and microstructure, directly affect the quality of final ceramic products. The molecular homogeneity of the chemical composition in the composite ceramic powders is also a significant factor for controlling the uniformity of final composite ceramics.
Barium titanate (BaTiO.sub.3), a perovskite-type electroceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. Two crystalline phases of barium titanate are especially important for applications in the microelectronics industry. The tetragonal phase of barium titanate show ferroelectric, piezoelectric, and thermoelectric properties and is used in a broad array of electronic devices. The cubic form of barium titanate, though not ferroelectric, has a high dielectric constant (1500-1600 at room temperature) making it suitable in capacitors. The wide applications of barium titanates include multilayer capacitors in electronic circuits, electro-optical devices, thermistors, piezoelectric actuators, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensitivity to gas adsorption. The overriding goal in barium titanate processing is to create smaller, ultrafine, more uniform particles and powders to allow for finer ceramic layers to be used in multlayer capacitors (and thus, achieve device miniaturization) without the loss of dielectric properties. Controlling the phase, composition homogeneity, particle size and monodispersity, microstructure, and even the cost of particle production are important concerns in developing techniques for synthesizing barium titanate.
Traditionally, barium titanate has been prepared by a solid-state reaction involving barium carbonate and titanium dioxide, typically at temperatures over 900.degree. C. as described by Niepce and Thomas, 1990, followed by grinding and further calcination. The resulting microstructure has not been sufficient for microelectronic applications due to a lack of uniform nanocrystalline barium titanate. The resulting microstructure has a wide grain-size distribution, multiple phases, and is inevitably porous. These characteristics result from inadequate existing ball-milling operations that can introduce such impurities as alumina, silica, sulfur, phosphorus, etc. The lack of control over the physical and/or chemical characteristics of commercial barium titanate particles results in microstructural variations that lead to poor electrical property optimization and reproducibility. Wet chemical techniques for synthesizing ceramics offer exciting ways for preparing ultrafine, homogeneous, high-purity powders at temperatures far below those required for conventional powder preparation.
Various wet chemical synthesis methods in literature for preparing barium titanate powders include homogeneous precipitation (described by Kim et al. 1996), coprecipitation, gas condensation (described by Li et al., 1997) and sol-gel processing (described by Kerchner et al., 1998). However, these techniques are not without some serious shortcomings. For instance, sol-gel processing with metal alkoxides allows for the fabrication of fine particles of barium titanate or other ceramics, but temperatures over 700.degree. C. are required to remove unreacted organics from the crystalline phase.
Wet chemical, hydrothermal processing with inorganic precursors offers an exciting way to prepare ultrafine (submicron), crystalline ceramics by using an aqueous medium under strongly alkaline conditions, as described by Kerchner et al., 1998. Numerous ceramic powders can be conveniently synthesized by the hydrothermal method, which allows for production of phase-pure products under low temperatures (.ltoreq.100.degree. C.) and facile control of reaction conditions such as concentrations, pII, and temperature. In contrast to the expensive sol-gel route with alkoxides, inorganic salts of barium and titanium used by Wada et al. (1995) and Eckert Jr. et al. (1996) are relatively inexpensive. Wada et al (1995) was able to produce barium titanate by adding barium hydroxide to a gel network of hydrous titanium tetrahydroxide, while Eckert Jr. et al (1996) prepared barium titanate from hydrothermal conversion of a commercial titania particles in solutions of barium hydroxide octahydrate. Two possible mechanisms have been generally proposed for the hydrothermal conversion of titania into barium titanate (Kerchner et al, 1998; Eckert Jr. et al., 1996; Hertl, 1988): (1) In-situ transformation, involving Ba.sup.2 + diffusion through formed BaTiO.sub.3 shell layer around the titania particle, then reaction with TiO.sub.2 core. The overall conversion rate could be either diffusion-controlled or reaction-rate controlled. Barium titanate particles obtained by this mechanism should maintain their size and morphology similar to those of their precursor titania particles. (2) Dissolution-precipitation, involving TiO.sub.2 dissolution into species Ti(OH).sub.x.sup.4-x, precipitative BaTiO.sub.3 nucleation (heterogeneous or homogeneous in nature) by reaction with Ba ions/complexes in solution, and then recrystallization/growth. Barium titanate particles from this mechanism are usually different from the precursor titania particles in their size and shape. The hydrothermal methods using titania gel or commercial titania particles are effective (kinetics and composition wise) in producing submicron barium titanate, however, synthesis of very uniform (also dispersed) microspheres has not been achieved and thus needs more development.